Pathogenic microorganisms can be introduced into the food
chain at any point, through agricultural conditions or contamination
during processing. Though proper heat treatments and subsequent sanitary
handling should keep most pathogens out of the food supply, they are
common in production environments of raw foods. Therefore, continuous
efforts are needed to effectively prevent pathogenic contamination and
Federal and state government agencies work together closely
to safeguard the food supply. From inspecting farms and slaughterhouses
to ensuring effective pasteurization and proper distribution temperatures,
these agencies have made the U.S. food supply one of the world’s
Even with these safeguards, according to the Atlanta-based Centers for Disease Control and Prevention (CDC), on average, someone in the United States develops a food-borne illness — an array of illnesses caused by consuming foods contaminated with certain microorganisms — every second. Though most are minor afflictions, CDC statistics show that these cause as many as 325,000 Americans to be hospitalized annually and about 5,000 to die each year. The numbers are much greater in other countries.
For example, today’s milk supply is so clean because
very few pathogenic bacteria can survive standard HTST pasteurization
temperatures. But the milk’s longer shelf life can give pathogens
time to grow to noteworthy levels, particularly if the milk is subjected
to temperature abuse. Years ago, inherent spoilage flora would have
soured milk long before any pathogen could increase to deleterious levels.
In addition, manufacturers’ efforts to make foods
more convenient and tasty also increase safety risks by going outside
traditional processing, packaging and even distribution parameters that
formerly ensured pathogens or their toxins would not contaminate foods.
Today’s consumers want faster food that tastes fresh; ready-to-serve
produce; and ready-to-eat (RTE) meat and poultry entrées with
a slow-cooked taste. They also want to have faith in the food supply.
Food manufacturers have the responsibility to use every tool available
to ensure a high-quality, safe food supply. Thankfully, scientists continue
to identify formulating, processing and packaging tools to help reduce
or eliminate undesirable microorganisms.
Though it’s imperative to use high-quality raw materials,
even the highest-quality food still contains microorganisms — spoilage
and pathogenic. So, formulators will often turn to ingredients that
lend antimicrobial assistance.
Antimicrobials reduce or eliminate microorganisms. Since the beginning of time, ingredients such as salt, sugar and vinegar have preserved food. Today, a host of other compounds prevent or retard microbial growth. The most-prevalent ones fall into these categories: proteins, organic acids and their salts, live cultures, plant-derived compounds, and certain foods.
Natamycin, also known as pimaricin, is an antifungal agent
produced by Streptomyces natalensis. The
compound is effective against yeasts and molds, but not bacteria. It
is approved in the United States for use as an additive to the surface
cuts and slices of cheese to inhibit mold spoilage.
Lysozyme, an enzyme derived from egg whites, has a bacteriocidal
effect on a range of lactic-acid spoilage bacteria, including Lactobacilli,
Leuconostoc and Pediococci. In
natural-cheese applications, it attacks Clostridium
tyrobutyricum, which produces undesirable gas that results in
late blowing of cheese. It also prevents flavor degradation without
affecting the cheese culture during aging. In December 2000, FDA recognized
lysozyme as GRAS as an antimicrobial agent in hot-dog casings, and on
cooked meat and poultry products. In all lysozyme applications, ingredient
panels must state “egg white lysozyme” to alert egg-allergic
consumers of its presence.
The bioactive milk protein lactoferrin plays an important
role in immune-system response and helps protect the body against infections.
It is an ingredient in infant formula, dairy products and chewing gums.
In January 2002, USDA approved activated lactoferrin for use on fresh
beef, providing beef processors with a potentially powerful technology
to protect consumers from pathogenic bacteria.
“Activated lactoferrin” describes a unique combination
of natural ingredients that mimic the optimum environment necessary
for lactoferrin’s maximum antimicrobial activity. Activation biases
lactoferrin to its iron-free and immobilized forms, in effect returning
lactoferrin to its most natural and functional state. A multiple-function
ingredient, activated lactoferrin protects meat in three key ways: it
detaches pathogens already attached to meat, prevents other pathogens
from adhering to meat and inhibits pathogen growth. It does all this
without impacting the meat’s taste, appearance or nutritional qualities.
Activated lactoferrin has a U.S. patent.
Researchers at the University of Alberta, Edmonton, have
developed an experimental proteinaceous compound that is sprinkled or
sprayed onto food and can neutralize some of the most-common food-borne
germs. Developed from freeze-dried egg yolk, the compound is an antibody
that works by binding with pathogens, carrying them through the body
without causing infection. “[It] does not kill the germs but prevents
them from infecting your body,” says Hoon Sunwoo, Ph.D., one of
the researchers, who recently presented his findings during the 225th
national meeting of the American Chemical Society in New Orleans. “The
antibody can remain active for one or two hours after being ingested.
That buys precious time that can help keep you alive if you eat contaminated
To make the compound, hens are injected with specific food-borne germs. They then develop antibodies to the germs as their immune system attempts to attack them, with the antibodies accumulating in large quantities in their eggs’ yolks. These eggs are processed and freeze-dried to form a natural, germ-fighting compound. The flavorless compound has a two-year shelf life and does not alter the foods’ taste.
Organic-acid ingredients for food preservation include
acetic, benzoic, citric, lactic, malic, propionic and sorbic, as well
as their soluble salts (calcium, potassium or sodium). Effective use
levels and target microorganisms vary by ingredient. For example, sorbic
acid and its soluble salts (such as potassium sorbate) are effective
against yeast and mold inhibition, with little activity against bacteria.
Common applications include cheese, sausage and baked goods, excluding
yeast-raised products. Calcium and sodium propionate are effective against
molds and have slight antibacterial action, but little action on yeasts,
so they work well in yeast-raised baked products.
Sodium benzoate is primarily used as an antifungal agent
in jams and jellies, fruit-flavored beverages, baked goods, and salad
dressings. Its use against bacteria is limited by poor activity above
pH 4.0, where bacteria are the greatest problem.
Antimicrobial agents formulated with sodium or potassium
lactate and sodium diacetate are effective in inhibiting the growth
of L. monocytogenes in RTE meat and poultry
On Dec. 9, 2002, FSIS put into effect directive 10,240.3,
which instructs inspection-program personnel on when and how often to
inspect RTE meat and poultry products, such as deli-type meats and poultry
that are sliced in the establishment or at retail, and hot-dog-type
products. The directive identifies antimicrobial agents formulated with
sodium/potassium lactate and sodium diacetate as a means to have the
RTE products classified as “low risk.” Establishments that
produce low-risk products may be eligible for FSIS’s low-targeted
verification testing program, meaning decreased frequency of product,
food-contact surface and plant-environment testing.
“It is advantageous for RTE meat and poultry processors
to be included in the low-verification testing program, because decreased
frequency of pathogen testing means lower costs for processors,”
says Jim Lees, market manager, meat and poultry, PURAC America Inc.,
Lincolnshire, IL. “establishments that produce high- or medium-risk
RTE products and that do not carry out regular environmental testing
for L. monocytogenes are subject to an
intensified verification testing program for product, food contact surfaces
and plant environment, resulting in higher costs for the processor.
“Purasal® Opti.Form™ is formulated from
natural sodium or potassium lactate and sodium diacetate,” says
Lees, and provides the right balance between flavor and effectiveness.
The liquid product, which is added directly to the brine for whole-muscle
products and at the final mixing stage for emulsified products, is highly
effective in controlling L. monocytogenes and Salmonella.
“By using it with RTE products, processors can obtain low-risk-product
classification,” he adds.
The company offers a predictive model to calculate the
levels of lactate and diacetate required to retard the growth of L.
monocytogenes in cured meat and poultry products. This scientific
model takes into account such factors as the amount of moisture in the
finished product, concentrations of sodium or potassium lactate, sodium
diacetate and salt, and GMPs. The model assumes a finished-product storage
temperature of 40ºF.
Acidified calcium sulphate, an organic acid and calcium sulfate combination identified by researchers at Texas A&M University, College Station, also shows promise as a way to kill L. monocytogenes in RTE meats, as well as certain cheeses. The researchers inoculated commercially made hot dogs with a four-strain L. monocytogenes cocktail containing 10 million microorganisms per gram, representing a worst-case scenario. Samples were treated with saline (control), acidified calcium sulphate, potassium lactate or lactic acid. The hot dogs were then vacuum-packed and stored under refrigerated conditions for 12 weeks. Researchers found the acidified calcium sulphate killed the surface Listeria and also stopped the organism from returning.
Formulators looking to decrease preservative additives
in foods to obtain a more “natural” label can turn to select
lactic-acid bacteria, which inhibit the growth of undesirable microorganisms
through their metabolites. In other words, it’s microorganism against
Because purified nisin is approved in the United States
for limited applications, an alternative approach to reap the bacteriocidal
benefits of nisin is to ferment certain foods with nisin-producing lactic-acid
bacteria. For example, if the lactic-acid bacteria used for fermented
dairy products, such as buttermilk, yogurt and sour cream, can synthesize
nisin or some other bacteriocin, the dairy product, in fact, has its
own built-in all-natural preservation system.
Now, take this a step further. Once the bacteriocin is
produced in these foods, they in turn can become ingredients in other
foods, indirectly adding the bacteriocin to the prepared food. Ingredient
labels can simply read “cultured milk.”
Cultured whey also can inhibit mold growth. Select food-grade
microorganisms ferment whey, resulting in an ingredient loaded with
natural preservatives. For example, one company ferments whey with propionic
bacteria, which produce calcium propionate and a small amount of acetate.
The whey ingredient — simply labeled as “cultured whey”—
has application in sauces, marinades, breads, sausages, jerky and other
Similar non-dairy mold inhibitors are also available. These use corn-syrup solids or dextrose as the fermentation substrate, and are listed accordingly on ingredient statements.
The hurdle concept uses several different antimicrobial
interventions to successfully reduce microbial risk to acceptable levels.
This includes ingredient additions, along with formulation adjustments
and process interventions. Combining a number of factors realizes the
preservative effects of each without impacting the food’s sensory
attributes. Additionally, this combination often exerts a synergistic
effect that produces greater efficacy against target microorganisms.
Microorganisms differ in their resistance to a given spice
or herb, with bacteria more resistant than yeasts and molds. Specifically,
Gram-negative bacteria are more resistant than Gram-positive bacteria.
Furthermore, the effect on spores may be different than on vegetative
The most-effective antimicrobial spices are cinnamon,
cloves, garlic, mustard, onion, oregano, sage and thyme. The essential
oil eugenol in cinnamon, cloves and sage possesses antimicrobial properties,
as does the allyl isothiocyanate present in mustard. Allicin in garlic
also acts as an antimicrobial agent and has been effective in controlling
E. coli. Thymol, found in thyme, oregano
and sage, is also noted for its antimicrobial properties.
Daniel Y.C. Fung, Ph.D., and other researchers at Kansas
State University, Manhattan, have studied the antimicrobial properties
of a variety of spices in meat products inoculated with E. coli O157:H7,
and found that cloves have a high antimicrobial effect against this
pathogen in ground beef. Cinnamon, garlic, oregano and sage also proved
Fung also looked at the relationship between garlic and
heat in E. coli-inoculated ground beef. He found that garlic provides
protection against the pathogen’s growth in undercooked ground
beef. Additionally, as the cooking temperature increased, so did garlic’s
Because garlic doesn’t complement all foods, Fung
investigated the antimicrobial properties of cinnamon in pasteurized
apple juice inoculated with E. coli O157:H7. Results show that
just 0.3% cinnamon inhibits E. coli O157:H7 in juice stored for
three days at 77ºF. The same amount is effective at 47ºF for up to eight
Studies from researchers at the University of Massachusetts,
Amherst, working with oregano extract in meat, have shown that a small
amount of oregano extract significantly slows the growth of Listeria.
Oregano has a powerful flavor, and though the amount used is small,
the group is trying to dilute the flavor to keep it from overpowering
the meat. Kalidas Shetty, Ph.D., a lead researcher on the project, notes
that eliminating the entire flavor is not possible because some of the
flavor intensity is linked to the compounds that inhibit Listeria
Researchers at Complutense University, Madrid, Spain,
recently discovered that dipping fruit in trans-resveratrol —
an antioxidant that is found in grapes — reduced yeast and mold
growth, thus extending shelf life. In this experiment, apples’
shelf life went from two weeks to three months, while the shelf life
of similarly dipped grapes went from one week to two weeks.
Trans-resveratrol is one of the red-wine components thought to combat heart disease and even cancer in moderate wine-drinkers, perhaps by neutralizing oxidizing agents, such as free radicals normally created in the body. Researchers theorize this same neutralizing effect probably prevents fruit-tissue damage, slowing the onset of mold and yeast growth.
In a follow-up study, they also evaluated dried-plum puree,
fresh plum-juice concentrate, and a powder — consisting of a mixture
of dried plums and pears — in similarly inoculated uncooked ground-beef
and pork-sausage samples. After five days, a 1 to 2 log cfu (colony-forming
units)/gram reduction of all inoculated pathogens occurred in the uncooked
ground beef containing dried-plum puree or plum juice. In the uncooked
pork sausage, significant suppression (at least 0.5 log cfu/gram) of
total aerobic count, E. coli O157:H7, L. monocytogenes,
Y. enterocolitica and S. aureus was observed with 6% dried-plum
puree and 6% dried-plum and pear powder.
Raisins also possess antimicrobial properties, related to their phenolic content. Golden raisins show the highest phenolic concentration, due to their lack of browning. Though many of the phenolics in dark raisins are lost because of browning reactions, the drying process used to produce raisins concentrates the remaining phenolics, making them significant on a per-weight basis.
Nitrate, the salt of nitric acid, and nitrite, the salt
of nitrous acid, are used for curing meat, poultry and fish products.
Nitrate must be chemically reduced to nitrite to produce the curing
reaction or exert antimicrobial properties. Nitrites can inhibit the
growth and toxin production of C. botulinum in cured products.
Some food manufacturers, though, avoid using nitrates and nitrites when
possible, as research has shown nitrites produce carcinogenic compounds.
Hydrogen peroxide is approved for use as an antimicrobial
in milk intended for the production of cheese, whey and dried-egg products.
Research shows liquid smoke can eliminate E. coli O157:H7, as well as other pathogens, in meat products. Its functionality is mainly due to organic acids, including acetic and propionic, which lower pH and destroy bacterial cell walls. Liquid smoke has the added benefit of being labeled or classified as a natural product.
HTST pasteurization destroys all pathogenic microorganisms
in food, along with many spoilage microorganisms. UHT pasteurization
destroys all viable microorganisms, creating a virtual sterile environment.
Though these techniques work well for many food and beverage applications,
manufacturers cannot pasteurize many high-risk products, or prefer not
to place them under intense heat treatment because of the negative effect
on sensory attributes.
Scientists at USDA Agricultural Research Service (ARS)
recently inactivated bacteria in apple juice through the use of radio-frequency
electric fields (RFEF). David Geveke, a chemical engineer at the Food
Safety Intervention Technologies Research Unit at USDA’s Eastern
Regional Research Center, Wyndmoor, PA, built a specially designed treatment
chamber to apply high-intensity RFEF to apple juice. Although RFEF has
been studied for more than 50 years as a pasteurization method, this
is the first confirmed instance of a successful inactivation of bacteria
using this technique in fruit juice. This nonthermal technique provides
an attractive option to conventional heat pasteurization, which destroys
nutrients and flavors in fruit and vegetable juices. However, when moderate
heat is applied, the combined effect is much greater than the effect
of either process alone.
Researchers conducted experiments using E. coli
K12, a harmless form of bacteria used to study similarly behaving pathogenic
strains, such as E. coli O157:H7. Apple juice was exposed to
electrical-field strengths of up to 20 kilovolts/cm and frequencies
in the range of 15 to 70 kilohertz, using a 4-kilowatt power supply.
Increasing the field strength and temperature, as well as decreasing
the frequency, enhanced inactivation. E. coli in a juice sample
at 122ºF was reduced 99.9%, suggesting that RFEF could provide an alternative
to traditional HTST pasteurization, which occurs at 161ºF. The technique
has potential with other heat-sensitive products, including liquid eggs.
Another alternative to heat pasteurization is high-pressure
processing (HPP). This technique reduces the vegetative microbial load
in foods by breaking hydrogen bonds, but not covalent bonds. This denatures
protein, which inactivates bacteria without changing vitamins, flavors
and colors. Combining heat with pressure can increase the treatment’s
Prepared, refrigerated guacamole was the first commercial
product in North America to use HPP. Avocadoes are low-acid and loaded
with enzymes, resulting in a very short shelf life for fresh guacamole
— about a week. HPP extends shelf life to 30 to 40 days, without
HPP also reduces pathogens without heat. Heat destroys
much of the avocado flavor and can cause oil separation in avocado-based
products, eliminating normal pasteurization as an option. HPP guacamole
requires high-barrier plastic packaging to prevent oxygen exposure,
as HPP does not destroy the enzymes; it just inactivates their activity
in oxygen’s absence.
Another nonthermal food preservation technique, pulsed
electric field (PEF) technology, applies a short burst of high voltage
through fluid foods placed or flowing between two electrodes. The treatment
is conducted at ambient or refrigerated temperatures for microseconds,
minimizing heat generation due to energy transfer. Foods retain fresh-like
physical, chemical and nutritional characteristics, and exhibit extended
refrigerated shelf life.
Packaging technology has advanced, too. The shelf life
of hard cheeses can increase two to nine months when packed in material
made from bio-based polymers. Researchers at The Royal Veterinary and
Agricultural University, Copenhagen, Denmark, have applied oxygen scavengers
and other preservatives to a packaging material made from polylactate.
The material is based on lactic acid produced by lactic-acid bacteria
found in corn. The packaging not only extends shelf life prior to opening
the cheese; it also improves the keeping quality once it’s opened.
The packaging’s active components reduce mold growth and development
of rancid taste.
Combining barrier packaging with an antimicrobial gas
flush creates an environment that reduces or prevents microbial growth.
This technique, referred to as modified-atmosphere packaging (MAP),
most often uses carbon dioxide gas, which inhibits a variety of microorganisms.
Its effectiveness depends on variables such as gas concentration, initial
contamination, water activity of the food product, packaging barriers,
Sulfur dioxide, another gas long used as an antimicrobial,
is effective against bacteria, molds and yeasts, and works in applications
including wine, dehydrated fruits and vegetables, fruits juices, syrups,
pickles, and fresh shrimp. It performs best at pH values below 4.0.
Various health concerns — including reported reactions to sulfur
dioxide by asthmatics, and headaches other consumers have associated
with sulfite consumption — have decreased its use by food manufacturers
in recent years.
Research shows that ozone acts as an effective fruit and
vegetable sanitizer. FDA recently approved commercial ozone use in U.S.
supermarkets and food-processing facilities. Application typically takes
place by washing produce with ozone-enriched water.
Ozone molecules contain three oxygen atoms and form when
oxygen molecules with two atoms are forced to take on a third. Ozone’s
use as a sanitizing agent comes from its unstable molecule structure
— the third oxygen atom tends to break apart from the ozone molecule,
releasing energy. When exposed to ozone, the produce’s surface
bacteria absorb the highly unstable molecules. When that third oxygen
atom breaks away, the bacteria explode.
Ozone-enriched water washes kill almost all the bacteria on fresh produce samples, along with some yeast and mold, increasing shelf life by up to two weeks. The treatment also retards softening and browning.
Irradiation exposes food, either prepackaged or in bulk,
to controlled levels of ionizing radiation — a type of energy similar
to radio and television waves, microwaves and infrared radiation. However,
the high energy produced by ionizing radiation allows it to penetrate
deeply into food, disrupting the genetic material of microorganisms,
thus destroying them.
Concerns about consumer acceptance for irradiation have
slowed its adoption by food processors. To combat this, the “First
World Congress on Food Irradiation: Meeting the Challenges of Food Safety
and Trade” seminar was held in Chicago from May 5-7, 2003. Organized
and sponsored by several groups, including the National Food Safety
& Toxicology Center at Michigan State University, East Lansing,
the congress is expected to lead to a wider acceptance and application
of food irradiation as a measure to ensure microbiological safety of
food and to facilitate food trade worldwide.
The number of supermarkets marketing irradiated food,
mainly ground beef, has grown from less than 100 stores in mid-2000,
when irradiated beef was introduced at the retail level, to more than
6,000 stores as of early March. The number of fast-food restaurants
serving irradiated hamburgers — including International Dairy Queen,
Inc., Minneapolis — and other chain restaurants serving irradiated
food is also growing rapidly. SureBeam Corp., San Diego, a provider
of irradiation technology, expects to process between 300 million and
350 million pounds of beef in 2003, a significant increase from the
15 million pounds it processed in 2002.
Irradiated food’s safety is well established by scientific
studies, including many long-term, multigeneration animal-feeding tests
conducted during the past five decades. More than 40 nations have approved
the use of food irradiation, and it is recognized by numerous professional
groups and food and health organizations around the world, including
the World Health Organization, American Medical Association and American
Dietetic Association, as a safe means of reducing the levels of organisms
that can cause food-borne illness and disease. FDA has approved irradiation
for beef, poultry, pork, eggs, fruits, vegetables, roots and tubers,
grains and legumes, spices, and other foods.
No foolproof technology to eliminate microorganisms from the food supply exists. But science is working with the food industry to furnish many options, allowing food manufacturers to give it their best shot and, hopefully, take full advantage of what science has to offer.
Donna Berry, president of Chicago-based Dairy & Food Communications, Inc., a network of professionals in business-to-business technical and trade communications, has been writing on product development and marketing for nine years. Prior to that, she worked for Kraft Foods in the natural-cheese division. Donna has a B.S. in food science from the University of Illinois in Urbana-Champaign. She can be reached at email@example.com.
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Reducing MicrobesJune 01, 2003 - Article
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